My understanding (which could be wrong) is that it also allows them to change the licence down the road.
For example, if you've contributed code to an GPL2-licenced project, and several years later the maintainer wants to change the project's licence to BSD, having a CLA means that the maintainer doesn't need to ask contributor individually.
For example, if you've contributed code to an GPL2-licenced project, and several years later the maintainer wants to change the project's licence to BSD, having a CLA means that the maintainer doesn't need to ask contributor individually.